Gaming machines include games of chance such as slot machines. The traditional mechanical slot machine includes three or four symbol-bearing reels rotatably mounted on a common axis. The symbols are located on the peripheries of the reels, and are typically pictures of bells, bars, and fruit. There are also "blank" symbols, which are the portions of the reels' peripheries in between the picture symbols. As used herein, "symbols" includes blanks.
The reels are placed behind a glass plate, and are oriented so that their symbol-bearing peripheries face the player. A horizontal win line is etched or painted on the glass plate so that when the reels are at rest, one symbol from each reel is visually associated with the win line. Thus, a "win line" defines the symbols that produce a single game outcome. For example, on a three reel slot machine having a single horizontal win line, the win line defines the three symbols that determine the game outcome. Some machines make use of the fact that more than one symbol from each reel may be visible to the player and therefore have multiple win lines, including both horizontal and diagonal lines. Thus, for example, on a three reel slot machine having three horizontal and two diagonal win lines, five groups of three symbols define five game outcomes, i.e., the player has five different ways to win.
To play the slot machine, the player spins the reels by pulling a handle which is mechanically linked to the reels. As the reels spin, they display a series or progression of symbols along the win line. After a brief period of spinning, each of the reels comes to rest at one of many predetermined and discrete "reel stop positions." At each reel stop position, a particular part of the reel's periphery, i.e., a symbol, is displayed at the win line. The game outcome is the particular combination of symbols displayed at the win line.
In the 1970's, manufacturers developed electronic versions of the traditional mechanical slot machine. In these electronic machines, the reels are computer controlled, and there is no mechanical linkage between the handle and the reels. Instead, when the user pulls the handle, the computer randomly selects reel stop positions for each of the reels, and then sets the reels into motion with a motor. The reels are allowed to spin for a short time, and then are stopped at the selected reel stop positions. In effect, the game outcome is determined by the computer, with the spinning reels used only to display that result. Thus, in some machines, the reels are eliminated altogether, and the game outcome displayed on a video screen. The video display is often a representation or facsimile of spinning reels, to preserve the charm and excitement of the traditional slot machine. To simulate the effect of a spinning reel, the video screen displays a series or progression of symbols, which appear to move past a win line.
When a machine has multiple win lines, each win line is capable of displaying a game outcome. It is possible for the player to select in advance which win line (the "active win line") will indicate the game outcome. Alternatively, several win lines may be active. The more active win lines, the greater the player's chances of obtaining a winning game outcome. This, in turn, increases the player's interest in the game.
One limitation of existing machines (even when multiple win lines are used) is that when the reels are at rest, each win line is associated with only one symbol from each reel. Thus, game outcomes only include combinations of symbols consisting of at most one symbol from each reel.